1. Field of the Invention
The present invention relates to a photomask, a lithography method and a method for manufacturing the photomask. In particular, the present invention pertains to an adjustment technology of an optical system of the projection lithography tool, installing a photomask on which a phase shifter is formed.
2. Description of the Related Art
Since photolithography patterns formed on a semiconductor wafer are continually becoming increasingly fine and precise, it becomes more and more important to precisely arrange the wafer in the focal point. Defocus caused by incorrect arrangement of the wafer leads to defective products. Therefore, methods for arranging the wafer in the focal point correctly have been developed. For example, asymmetrical diffracting gratings are employed. The asymmetrical diffracting gratings have asymmetrical diffracting portions, which generate phase different of light reaching value that is larger than 0 degrees and lower than 180 degrees. Such asymmetrical diffracting gratings generate a positive first order diffracting light and a negative first order diffracting light with different diffraction efficiencies.
When asymmetrical diffracting gratings are exposed with laser light, projected images on the wafer of the asymmetrical diffracting gratings displace perpendicularly to the optical axis if the position of the wafer is varied in the direction of the optical axis. The displacement of the projected images and the variation of the position of the wafer have a linear relationship. Therefore, once the linear relationship is obtained, it is possible to calculate wafer position from measured position of projected images. However, earlier methods require preparing two photomasks, one for monitoring of wafer position, and another for exposing a device pattern on a wafer. This is because it is difficult to fabricate both an asymmetrical diffracting grating and a device pattern on single mask substrate. Therefore, preparing two photomasks raises development costs, and the exchanging of photomasks interrupts continuous production. Further, defocus failure may be caused while the photomasks are exchanged.